1. Field of the Invention
This invention relates to carbide-free bainitic steel rails and to methods of producing such steel rails. More especially, but not exclusively, the invention relates to carbide-free bainitic steels having enhanced wear resistance and rolling contact fatigue from which inter alia track and crane rails, railway points and crossings.
2. Description of Related Art
Most track rails have hitherto been produced from pearlitic steels. Recent reviews have indicated that pearlitic steels are approaching the limit of their material property development for track rails. There is therefore a need to evaluate alternative types of steel having good wear and rolling contact fatigue resistance coupled with improved levels of ductility toughness and weldability.
EP 0612852A1 discloses a process for manufacturing high-strength bainitic steel rails having good rolling-contact fatigue resistance in which the head of the hot-rolled rail is subjected to a discontinuous cooling programme which entails accelerated cooling from the austenite region to a cooling stop temperature of 500.degree. to 300.degree. C. at a rate of 1.degree. to 10.degree. C. per second, and then cooling the rail head further to a still lower temperature zone. The bainitic steel from which the rails are produced is not carbide-free. Rails produced by this process were found to wear away more readily than conventional pearlitic rails and exhibited an improved resistance to rolling-contact fatigue. Thus, the increase in wear rate exhibited by the head surfaces of these rails ensured that accumulated fatigue damage wore away before defects occurred. The physical properties exhibited by these rails are achieved in part by the accelerated cooling regime referred to above.
The solution proposed by EP 0612852A1 differs markedly to the method of the present invention which achieves in rail steels substantially enhanced wear resistance with excellent resistance to rolling-contact fatigue. These steels also show improved impact toughness and ductility in comparison with pearlitic rails. The method of the present invention also avoids the need for a complicated discontinuous cooling regime as specified in EP 0612852A1.
Other similar documents specifying complicated discontinuous cooling regimes include GB 2132225, GB 207144, GB 1450355, GB 1417330, U.S. Pat. No. 5,108,518 and EP 0033600.
Track rails produced from iron carbide containing bainitic steels have been proposed previously. Whereas the fine ferrite lath size (.about.0.2-0.8 .mu.m wide) and high dislocation density of continuously cooled bainite combine to make the steels very strong, the presence in the microstructure of inter and intralath carbides leads to increased embrittlement which has to a large extent tended to hinder commercial exploitation of such steels.
It is known that the embrittlement problem which occurs because of the presence of deleterious carbides can be largely alleviated by employing relatively large silicon and/or aluminium additions (.about.1-2%) to low-alloy steels. The presence of silicon and/or aluminium steels continuously transformed to bainite encourages the retention of ductile high carbon austenite regions in preference to the formation of brittle intralath cementite films, and depends on the premise that the dispersed, retained austenite should be both thermally and mechanically stable. It has been shown that the retained austenite following continuous cooling transformation in the bainitic temperature range occurs either as finely divided thin intralath films, or in the form of "blocky" interpacket regions. While the thin film morphology has extremely high thermal and mechanical stability, the blocky type can transform to high carbon martensite, less conducive to good fracture toughness. A ratio of thin film to blocky morphology &gt;0.9 is required to ensure good toughness, and this can be achieved through a careful choice of steel composition and heat treatment. This results in an essentially carbide free, "upper bainite" type microstructure based on bainitic ferrite, residual austenite and high carbon martensite.